The present invention relates to a method of improving middle distillate fuels. More specifically the present invention relates to a method of selectively incorporating oxygen into diesel fuels in order to improve emissions characteristics by reducing the level of particulates and/or increasing the cetane number of the diesel. Improving cetane number of a diesel fuel results in improved ignition characteristics such as improved cold weather starting, reduction in ignition delays, combustion noise and misfiring.
Previous approaches to improving the cetane number of diesel fuel have included blending with higher cetane value streams, hydrotreating and/or the addition of cetane enhancing additives. These approaches suffer from cost/availability issues for hydrogen and the cetane enhancing additives. A desirable approach would be to carry out a heterogeneous catalytic process that results in the selective oxygenation of the fuel without the addition of expensive chemical oxidizing agents such as organic peroxides, ozone or hydrogen peroxide.
In this connection U.S. Pat. No. 4,723,963 (Taylor) discloses a middle distillate hydrocarbon fuel comprising at least 3 weight percent oxygen. Taylor teaches the selective oxygenation of hydroaromatic and aromatic compounds by passing oxygen and/or air through the compounds or by the use of chemical oxygen donor compounds or by reacting the compounds to form halides followed by the hydrolysis to form the alcohol or dehydrogenating the compounds to form olefins and reacting the olefinic aromatics with water, or with carbon monoxide and hydrogen. This oxygenated stream can then be blended with a paraffinic rich stream.
WO 01/32809 discloses another process for selectively oxidizing distillate fuel or middle distillates. The subject reference discloses that oxidized distillate fuels wherein hydroxyl and or carbonyl groups are chemically bound to paraffinic molecules in the fuel results in a reduction of particulates generated upon combustion of the fuel versus unoxidized fuel. The reference discloses a process for selectively oxidizing saturated aliphatic or cyclic compounds in the fuel with peroxides, ozone or hydrogen peroxide such that hydroxyl or carbonyl groups are formed in the presence of various titanium containing silicon based zeolites.
U.S. Pat. No. 4,494,961 (Venkat et al.) discloses a method of increasing the cetane number of a low hydrogen content highly aromatic distillate fuel by subjecting it to catalytic partial oxidation. The subject method involves heating the aromatic diesel fuel under mild oxidation conditions in the presence of a catalyst system comprising (1) an alkaline earth metal permanganate, (2) an oxide of a metal of Groups IB, IIB, IIIB, IVB, VB, VIB, VIIB, or VIII of the Periodic Table, or (3) a mixture of (2) and an alkali metal or alkaline earth metal oxide or salt.
An earlier effort to improve diesel fuel combustion characteristics by attaining minimum engine knocking i.e., the time interval between the instant of liquid fuel injection and the instant of ignition, is disclosed in U.S. Pat. No. 2,521,698 (Denison, Jr. et al.) The subject reference discloses a process that involves partial oxygenating of distillate by contact with an oxygen-containing gas whereby the fuel's cetane number is increased while not increasing the conversion to compounds that produce corrosion.
European Patent Application 0 293 069 discloses a fuel production process whereby the cetane number is improved by hydrogenating a naphthalene or alkylnaphthalene hydrocarbon oil to tetralin and partially oxidizing the hydrogenated oil to yield a hydrocarbon oil containing tetralin hydroperoxide. The partial oxidation is carried out by placing the oil under oxygen under pressure of 3 to 8 kg/cm2 at a temperature of 60 to 100 C. for a period of 3 to 10 hours or by adding a copper or nickel catalyst to the oil.
As is evident from the above discussion that what is needed is a process for increasing the cetane number of a distillate fuel via a direct oxygen incorporation from air or another suitable oxygen-containing gas without the addition of expensive chemical oxidizing agents or the time intensive, hence capital intensive contact periods while concomitantly not increasing corrosion by increasing the TAN acidity of the fuel.
The process of the present invention provides a relatively simple process for incorporating oxygen into middle distillate or diesel range hydrocarbon feedstocks by contacting the feedstock with an oxygen-containing gas in the presence of a heterogeneous catalyst comprising a Group VIII metal on a basic support.